There is not a *set* number of Dragons. There are 20 dragon shouts comprised of three words each. Leading to 60 words. You learn these words in various ways including ancient texts written deep in dungeons. They have said many times that dragons are not scripted. As in they fly into battle at unexpected times and are a constant threat.
From the Draogon Shout GI hubdate:
In the game, players will guide their hero to learn ever more powerful dragon shouts, and then use these arcane powers to supplement other combat and magic skills. Upon defeating a dragon, Skyrim’s hero absorbs the soul of the fallen creature, which fuels his ability to learn a new shout. Later, players can search out long lost walls covered in dragon script. Upon these walls, individual runes stand out to the hero because he or she is dragonborn. “There are these words of power, and if you learn how to say them right, they have a powerful effect,” Howard says.
Killing dragons fuels your
ability to learn new shouts. It's the equivalent of "leveling up" your Dragon Shout skill. Then you learn the shouts themselves, words of power, by finding the ancient runes.